The present invention relates to a method of making balls or spherical bodies of high density from powder by way of rotation, and particularly to a method for making or rather manufacturing small spherical bodies made of inorganic material such as silicon nitride ceramic balls in a low cost mass-production by way of rotation. This invention also relates to spherical bodies of high density and particularly to ceramic balls for use in various ball-using applications, made by the method of the invention. This invention further relates to an adjusted powder composition that is suitably used for the method of the present invention to make the spherical bodies.
Ceramic balls (or spherical ceramic bodies), particularly silicon nitride ceramic balls, can be incorporated into bearings requiring high duty, high durability, high speed and/or high precision. This is because ceramic balls are advantageous in improving the performance of a bearing in view of their hardness and light weight, as compared to metal balls. On the other hand, because ceramic balls for use as bearings are expensive, they have not yet been widely used nor popularly accepted across the total industry employing a very large number of high-performance balls.
One reason for the expense of the ceramic balls is that a low-cost method for forming ball-shaped or spherical sintered bodies having a high density from a ceramic powder has not yet been established, which requires a performance at least similar to or comparable with the metal balls for bearings.
Conventionally, a molding method has been employed for manufacturing relatively uniform and less defective spherical ceramic bodies so long as their diameter is relatively large. However, due to its low molding efficiency, this conventional molding process is not suited for inexpensively manufacturing a large number of small balls, for example, those for use in most bearings. For example, computer hard disk drive motors or other spindling devices require a large number of small bearing balls having a diameter of, e.g., 2 mm or the like, mostly less than 4.5 mm.
A die-pressing method using press punches requires two hemispheric cavities formed in their end faces. Such a die-pressing method unavoidably causes the formation of an unnecessary circular flange or brim portion around a press-formed spherical body due to unilateral movement of the punches, and tends to form a low density unfired spherical ball that possibly leaves pores or areas of non-uniform density inside the ball after sintering. The unnecessary portion has to be removed before firing an unfired ball or before polishing the fired body, adding more cost and inefficiency in manufacturing the ceramic balls.
Other known methods or processes for manufacturing spherical bodies from powder include a tumbling granulation process, a stirring granulation process and a squeezing granulation process. These processes require a material powder having large agglomerates ranging from 50-200 xcexcm with a high powder fluidity, normally made by spray-drying a slurry containing inorganic powder. On the other hand, a high performance ball bearing requires balls of fine surface. Namely, a fine ceramic powder having small agglomerates with a low fluidity is desirably employed for making ceramic bearing balls with uniform density and without defects such as discontinuous boundaries, holes and pores inside or on the ball surface, which increases reliability of the bearings.
The spray-dried powder material having such a large agglomerate size of from 50-200 xcexcm, which tends to induce non-uniform shrinkage, deformation, cracks or chipping during or after sintering unfired ceramic bodies, has been used in manufacturing relatively large-sized balls such as ball-media, e.g., for use in a cement-powdering process that does not require the high reliability or durability.
It is therefore an object of the present invention to provide a low-cost method for making spherical bodies from a powder, suitable for mass-production of spherical bodies. One of the features of the method of the present invention is directed to the use of a nucleus which forms and grows a granular body around the nucleus by rotating the nucleus. Another feature of the present method is directed to the use of an adjusted powder suitably adjusted for forming and growing a granular body having the nucleus inside the body during the rotation.
Specifically, the method of making spherical bodies from powder according to one aspect of the invention, comprises:
(1) preparing an adjusted powder having at least one powder characteristic selected from the group consisting of an average particle size, a powder particle size distribution and a BET specific surface area;
(2) preparing nuclei having a particle size larger than that of the adjusted powder;
(3) rotating the nuclei; and
(4) adding the adjusted powder and a solvent to the rotating nuclei so that particles of the adjusted powder accumulate on the nuclei to form granular bodies. Preferably, each granular body has the nuclei therein. The rotation of the nuclei may include a meaning of rolling. or tumbling the nuclei, i.e., rotation of the nuclei with some vibration added to the nuclei.
An advantage of the method of the present invention is that a large quantity of spherical inorganic bodies (e.g., bearing balls) of ceramic such as nitrides including silicon nitride, aluminum nitride or boron nitride, oxides including zirconia or alumina, carbides including tungsten carbide or titanium carbide, and the like, are inexpensively and efficiently manufactured.
Preparing the adjusted powder in the above method can advantageously comprise mixing at least an inorganic powder and a sintering aid powder uniformly to form a powder mixture having powder characteristics comprising an average particle size of 0.1-2 xcexcm, a 90% powder particle size distribution falling within a range of from 0.7-3.5 xcexcm, and a BET specific surface area of from 5-20 m2/g.
An advantage of this additional step is that a large quantity production of spherical green bodies, namely, spherical unfired balls, is facilitated. Another advantage is that the spherical bodies unfired or sintered have a high uniform density and are produced uniformly with less defects in a large quantity by virtue of using the adjusted powder in the method according to the invention.
An alternative of the above for preparing the adjusted powder comprises steps of
(1) forming a slurry by uniformly mixing at least a ceramic powder, a sintering aid powder and a solvent;
(2) drying the slurry into agglomerates; and
(3) pulverizing the agglomerates into a fine powder having an average particle size of 0.1-2 xcexcm, a 90% powder particle size distribution falling within a range of from 0.7-3.54 xcexcm determined according to a cumulative relative frequency distribution of powder particle sizes, and a BET specific surface area of 5-20 m2/g.
The advantage of these steps is directed to the slurry. This is because the slurry forms a uniform distribution of the ceramic powder and sintering aid powder mixed in the slurry and further forms aggregates comprised of the ceramic powder particles uniformly distributed with the sintering aid powder particles.
Various kinds of solvents may be used as long as pulverization of the aggregates can be accomplished. Water is recommended for the solvent, because water does not cause an environmental issue.
The method of making spherical bodies by rotation or tumbling according to the invention may further comprise:
(1) firing spherical granular bodies to thereby produce sintered spherical bodies; and
(2) grinding surfaces of the sintered spherical bodies to produce polished spherical bodies. This is because spherical bodies generally need a certain hardness and a surface smoothness in actual use.
In one aspect of the embodiments according to the invention, a method is provided for manufacturing spherical sintered ceramic bodies from ceramic powder of silicon nitride. The method comprises preparing an adjusted powder which predominantly comprises silicon nitride powder and a suitable amount of a sintering aid powder, wherein the adjusted material powder has at least, an average particle size of 0.1-2 xcexcm, more preferably 0.3-2 xcexcm, and a 90% cumulative relative frequency in powder particle size distribution of from 0.7-3.5 xcexcm, both as preferably measured using a laser diffraction granulometer, as well as a BET specific surface area of from 5-20 m2/g, more preferably from 5-13 m2/g; forming the adjusted powder into spherical green bodies by rotating nuclei adhering the adjusted powder; and sintering the spherical green bodies to obtain spherical sintered bodies of silicon nitride.
Herein, the 90% powder particle size distribution is the particle size value when Nrc (i.e., relative cumulative frequency) becomes 90% as understood by reference to FIG. 12(b), or to the particle size value when a Nrc value of 90% is attained or when the number Nc (i.e., cumulative frequency of particle size classes counted from the smallest particle size class) is divided by No (i.e., total frequency counted from the smallest particle size to the maximum particle size) as may be more clearly understood by referring to FIG. 12(a).
The average particle size can be expressed as a 50% particle size herein and defined as the Nrc value of 50% of that attained in a manner similar to the above, i.e., the average particle size means the particle size value when Nrc=50%.
Another object of the invention is to provide an adjusted powder suitably adjusted in use for the method of the invention, namely, making spherical balls in mass-production.
A feature of the adjusted powder is directed to adjustment of an original powder material. Namely, the adjusted powder adjusted from the original powder material has at least one of three powder characteristics including an average powder particle size, a relative cumulative frequency of powder particle size distribution and a BET specific surface area of the powder. An advantage of using the adjusted powder is that it facilitates adhering powder particles around the nucleus to form a spherical granular body and further adhering the powder particles to the granular body to grow the granular body further in size during rotation of the nucleus, with addition of a solvent such as water to the nucleus in rotation.
In one aspect of the invention, the best result of making spherical bodies is attained by use of an adjusted powder having all the above powder characteristics of an average powder particle size, relative cumulative frequency and BET specific surface area, in the method of the present invention.
These powder characteristics are important factors in forming the spherical granular body by rotation or tumbling according to the invention. Specifically, when the average power particle size, relative cumulative frequency of powder sizes and BET specific surface area are in the range of 0.1-2 xcexcm, 0.7-3.5 xcexcm at a 90% relative cumulative frequency, and 5-20 m2/g, respectively, a good result is attained in forming unfired spherical granular bodies and spherical sintered bodies made by the method according to the invention.
The average powder particle size and/or relative cumulative frequency of powder size distribution of the adjusted powder are determined by a laser granulometer, and the BET specific surface area of the adjusted powder is determined by a Brunauer-Emmett-Teller method.
Still Another object of the invention is to provide a spherical ball unfired or sintered, which is made from the adjusted powder by any of the methods according to the invention.
In one aspect, a spherical ball made according to the method of the invention have a high relative density. This is because the adjusted powder uniformly and firmly adheres around the rotating nucleus and around the rotationally growing granular body having the nucleus in the center due to a radial directional force being applied to the nucleus as well as the granular body by rotation.
The relative density of the spherical body in a unfired state becomes no less than 61% and as high as 75%, rendering more than 98% or rather very close to a 100% relative density to the resultant spherical sintered body when the unfired spherical granular body is subjected under a HIP (hot isostatic pressing) or a GP (gas pressure) sintering condition, in the case of, e.g., a ceramic spherical body, particularly a silicon nitride ceramic spherical body.
In one aspect, the density of the spherical body becomes high and uniform in a circular direction inside the spherical body made by the method of the invention as well as in a radial direction. This is because fine particles of the adjusted powder are forced into gaps among the particles adhered onto the granular body surface by a force in the radial direction of the spherical granular body.
A layer of the adjusted powder adhered on the granular body surface is formed and packed uniformly in the circular direction as well as in the radial direction during rotation of the granular body that has the nucleus therein.
This circular uniformity in addition to the radial uniformity of the density of the spherical body is advantageously important because the spherical body unfired or fired is protected from the formation of interior defects such as pores and holes. The resultant sintered spherical body is protected from cracking in a radial direction when it is actually used in an application such as a bearing. If there is such a defect inside or particularly close to or on the ball surface, high reliability and/or high performance of the bearing are not attained. This is because the ball in actual use for the bearing receives a high load in the radial direction possibly causing breakage or cracking leading to a failure.
A feature of the spherical bodies (or balls) according to the invention is as follows. A lightness and/or a chromaticity observed on an inside surface that is made by cutting the spherical ball across the ball center into hemispherical halves and polishing the inside surface of the half vary less in the circular direction but vary more in the radial direction of the spherical body between the center and the surface. The lightness and chromaticity are clearly identified as circular stripes in the case of unfired green spherical bodies. The variation in lightness and/or chromaticity in the radial direction means uniformity of such lightness and/or chromaticity in the circular direction. The variation of lightness and/or chromaticity in a radial direction and the uniformity of lightness and/or chromaticity in a circular direction are often observable more or less on the polished surface on the cut surface of the sintered spherical body, although the clear stripes as seen inside the unfired spherical body disappear in the sintered body.
In still another aspect, the spherical body made from the adjusted powder and then sintered in the HIP or GP, according to the invention, can be made as having substantially no inside pores between the nucleus and the surface of the spherical sintered body. This is because of the combination of the adjusted powder described herein, the method of the invention and a pressure-applied sintering process including HIP or GP.